A document by William S. D. Wilcock. Click on the document to view its contents.

The boundary between the overriding and subducting plates is locked along some portions of the Cascadia subduction zone. The extent and location of locking affects the potential size and frequency of great earthquakes in the region. Because much of the boundary is offshore, measurements on land are incapable of completely defining a locked zone in the up-dip region. Deformation models indicate that a record of seafloor height changes on the accretionary prism can reveal the extent of locking. To detect such changes, we have initiated a series of calibrated pressure measurements using an absolute self-calibrating pressure recorder (ASCPR). A piston-gauge calibrator under careful metrological considerations produces an absolutely known reference pressure to correct seafloor pressure observations to an absolute value. We report an accuracy of about 25 ppm of the water depth, or 0.02 kPa (0.2 cm equivalent) at 100 m to 0.8 kPa (8 cm equivalent) at 3,000 m. These campaign survey-style absolute pressure measurements on seven offshore benchmarks in a line extending 100 km westward from Newport, Oregon from 2014 to 2017 establish a long-term, sensor-independent time series that can, over decades, reveal the extent of vertical deformation and thus the extent of plate locking and place initial limits on rates of subsidence or uplift. Continued surveys spanning years could serve as calibration values for co-located or nearby continuous pressure records and provide useful information on possible crustal deformation rates, while epoch measurements spanning decades would provide further limits and additional insights on deformation.

We present observations from two field deployments of a calibrated tiltmeter that we name the Self-Calibrating Tilt Accelerometer (SCTA). The tiltmeter is based upon a triaxial quartz crystal accelerometer; the horizontal channels measure tilt and are periodically rotated into the vertical to obtain a measurement of the acceleration of gravity. Changes in the measured total acceleration are ascribed to drift in the vertical channel and used as calibrations for removing that same drift from the tilt time series observed between calibrations. Changes in the span (sensitivity) of the accelerometer channels can also be measured by calibrating them pointing up and down. A 3-year test on the seafloor at Axial Seamount show that the calibrations are consistent with a linear-exponential model of drift to ~0.5 μg (μrad). The calibrated tilt time series was impacted by platform settling for the first 2 years, but after repositioning the tiltmeter, the calibrated observations were consistent for the final year with the tilt observed on a nearby LILY tiltmeter. A separate 15-month test in a stable vault at Piñon Flat Observatory was complicated by seasonal temperature variations of >5°C; the calibrations are consistent with a linear-exponential model of drift to ~2 μg when temperature and temperature time-derivative dependence is included. Similarly, the calibrated tilt time series was impacted by thermal deformation of the SCTA assembly. A future test in a thermally and tectonically stable borehole will be required to assess the accuracy of the SCTA.

A document by Kaiwen Wang. Click on the document to view its contents.

Axial Seamount is a basaltic hot spot volcano with a summit caldera at a depth of ~1500 m below sea level, superimposed on the Juan de Fuca spreading ridge, giving it a robust and continuous magma supply. Axial erupted in 1998, 2011, and 2015, and is monitored by a cabled network of instruments including bottom pressure recorders and seismometers. Since its last eruption, Axial has re-inflated to 85-90% of its pre-eruption level. During that time, we have identified eight discrete, short-term deflation events of 1-4 cm over 1-3 weeks that occurred quasi-periodically, about every 4-6 months between August 2016 and May 2019. During each short-term deflation event, the rate of earthquakes dropped abruptly to low levels, and then did not return to higher levels until reinflation had resumed and returned near its previous high. The long-term geodetic monitoring record suggests that the rate of magma supply has varied by an order of magnitude over decadal time scales. There was a surge in magma supply between 2011-2015, causing those two eruptions to be closely spaced in time and the supply rate has been waning since then. This waning supply has implications for eruption forecasting and the next eruption at Axial still appears to be 4-9 years away. We also show that the number of earthquakes per unit of uplift has increased exponentially with total uplift since the 2015 eruption, a pattern consistent with a mechanical model of cumulative rock damage leading to bulk failure during magma accumulation between eruptions.

Orca seamount is located in the Bransfield Strait, between the South Shetland Islands and the Antarctic Peninsula. The volcano developed on an extensional rift produced by a combination of slab rollback at the South Shetland trench and transtensional motions between the Scotia and Antarctic plates. From January 2019 to February 2020, the BRAVOSEIS project deployed a dense amphibious seismic network in the Bransfield region, comprising both land and ocean bottom seismometers (OBS), as well as moored hydrophones. We perform an analysis of the seismicity recorded in the area of Orca volcano using a subnetwork composed of 15 OBS around Orca seamount and its SW rift, covering a region of about 20 km x 10 km, with inter-station distances of ~4 km. OBS data are organized, visualized and analyzed using the SEISAN software package. Earthquake detection was achieved through an STA/LTA algorithm. We use a visual procedure including spectral analysis and filtering to discriminate local earthquakes from other types of signals. For earthquake location, we use P and S phase arrivals and a layered model derived from previous geophysical studies in the region. In this way, we identify and locate around 3000 earthquakes with magnitudes in the range from -1 to 3. There is a continuous background level of microearthquake activity, although a large part of the earthquakes occurred during a swarm in June-July 2019. Source depths are mostly concentrated in the first 10 km (within the crust). The epicentral distribution covers the whole area around the volcano, but it is clearly densest in the NE flank, where an intense seismic series started in September 2020.

The Ocean Observatories Initiative Cabled Array (OOI-CA) is a 25-year facility that provides unprecedented power and communications at Axial Seamount and sites on the Cascadia continental margin. Although the OOI is sometimes viewed as too expensive, the CA provides two capabilities that would be infeasible in autonomous configurations. First, at the expense of a reduced number of instrument sites, it enables reliable long-term observations with suites of standard sensors. Second, it can support novel instrumentation with exacting power, bandwidth and real time requirements. At the summit of Axial Seamount, the core sensor network spans the southern half of the caldera, and comprises 7 seismometers, 2 hydrophones, 4 bottom pressure and tilt instruments, and a variety of sensors in two hydrothermal fields. Additional sensors have also been added for testing. The volcano erupted in April 2015, within months of the CA coming online, providing new insights into the workings of caldera ring faults and support for the inference that eruptions at Axial occur at predictable levels of inflation. As of Summer 2019, the volcano has recovered 70% of the deflation that occurred in 2015. Another eruption is expected in a few years and several more are quite possible over the lifetime of the system. Because Axial has a shallow magmatic system that is well imaged, it is an excellent setting to study the links between volcanism and caldera dynamics and search for signals that are precursory to eruptions at a variety of timescales. Such studies would benefit from temporary deployments of autonomous sensors timed to coincide with the predicted times of eruptions. The CA should also be used to address questions related to the formation of hydrothermal event plumes and their role in flushing out fluids and microbes from the subsurface. This will require the addition of one or more cabled moorings above the caldera and enhanced sampling capabilities. There is a proposal to install several instrumented boreholes at the summit of Axial Seamount that would employ the CA to support novel interactive microbial and hydrologic subseafloor experiments. The CA can also be used to test emerging technologies such as resident autonomous underwater vehicles and distributed acoustic sensing, which will provide new tools to address key scientific questions.

We report on a feasibility study for an offshore instrument network in the Cascadia subduction zone to improve earthquake and tsunami early warning. The global DART buoy network provides effective warning for far-field tsunamis but near-field tsunami warning is challenging because the lead time is short and near-source observations are rarely available to directly measure the sea surface disturbance and evolution. Near-field tsunami warnings presently rely on rapid point source seismic inversions that do not estimate tsunami wave height. Efforts are underway to incorporate GNSS data into rapid source inversions that would support an initial near-field tsunami prediction. Offshore observations would contribute further to near-field tsunami warnings by providing: first, direct observations of seafloor and sea surface displacements during earthquake rupture and second, ongoing measurements for continued forecast refinement. Offshore instruments could also detect tsunamis triggered by submarine landslides and by so-called “tsunami” or “slow” or “silent” earthquakes that can generate unexpectedly large tsunamis but are characterized by shaking intensity so low as to be undetected or ignored. Pressure observations in the source zone will be challenging to interpret because they are dominated by seafloor accelerations and hydroacoustic waves rather than changes in hydrostatic pressure. In an effective system, pressure observations may need to be complemented by other observations such as inertial measurements of seafloor displacement, GNSS buoys and high-frequency coastal radar. It may also be important to place pressure sensors just seaward of the source zone to measure the developing tsunami in a region with an undisturbed seafloor. We will discuss alternative design options for an offshore instrument network in Cascadia, the research and development that must to be completed to determine the best approach, and the role of offshore observations in a holistic plan for tsunami mitigation.

This preprint has been deleted as it was posted before approval by USGS.

Measurements of ground tilt are a critical geodetic tool for monitoring active volcanoes because they provide multidimensional data that can resolve complex deformation signals. We are developing a Self-Calibrating Tilt Accelerometer (SCTA) for use in the marine environment and present results from two deployments: on land at the Scripps Institution of Oceanography Cecil and Ida Green Piñon Flat Observatory and on the seafloor at Axial Seamount on the Juan de Fuca Ridge. The SCTA utilizes a Quartz Sensor Solutions triaxial accelerometer on a gimbal system to periodically rotate the horizontal channels into the vertical to calibrate against the local g vector, achieving high precision and stability within 1 microradian. The SCTA tiltmeter has the added benefit of simultaneously measuring ground accelerations and recording seismic signals. We compare the SCTA performance at the center of the summit caldera at Axial Seamount against a co-located Jewell Instruments LILY tiltmeter on the OOI Cabled Array. The tilt measurements in one direction are consistent, but the data suggest that the deployment platform for the SCTA may be settling in the other direction. We are using data from the ensemble of 4 cabled pressure sensors and 5 tilt sensors at Axial, including the SCTA, to study its inflation behavior since its eruption in 2015. We have identified several significant, cm-scale deflation events of durations of tens of days. The tilt and relative elevations of instrument sites are asymmetric about their turning points, suggesting a more complex mechanism than a simple inflation reversal. We are conducting forward modeling of the deformation signals to determine if the geodetic signals are consistent with differential slip rates, normalized to the rate of inflation/deflation, on the caldera’s outwardly dipping ring faults between these periods. Another plausible mechanism that we plan to investigate is the lateral transport of magma from beneath the southern caldera to either the northern caldera or to a secondary reservoir, located 5 km to the east. These deflation events are potentially important for understanding the mechanisms of magma supply, storage, and transport at Axial Seamount, as well as for accurately forecasting future eruptions, which have been shown to be inflation-predictable.

We use ocean bottom seismometer data from the Endeavour segment of the Juan de Fuca ridge to construct a long-term earthquake catalog for an intermediate-rate spreading ridge. We present > 50,000 new earthquake locations for 2016-2021 from the Ocean Networks Canada NEPTUNE cabled observatory and relocate earthquakes from two autonomous networks in 1995 and 2003-2006. The catalog comprises > 85,000 earthquakes located using three-dimensional segment-scale P- and S-wave velocity models from a prior tomography experiment. Despite the small footprints of networks near the segment center, locations show good agreement with geologic features at segment ends. The improved locations show that the northern Endeavour segment ruptured southwards from 48.3°N to 48.05°N during two diking events in early 2005, possibly accompanied by diking on the West Valley propagator. Persistent off-axis seismicity near the segment center appears to be related to the West Valley and Cobb propagating rifts which we infer extend ~10 km closer to the Endeavour segment center than is apparent in bathymetry. We suggest that the proximity of the propagators to the Endeavour vent fields contributes to the localization, vigor and longevity of the fields by focusing permeability through ongoing fracturing and by limiting extrusive magmatism through degassing of the axial magma lens. Increasing rates of seismicity beneath the vent fields beginning in late 2018 and a deepening of earthquakes in 2020 indicate that the central portion of the segment may be entering the later stages of the eruptive cycle.